A sensor is a measurement device placed in the vicinity of an enclosure in which flow or are contained gases, liquids or whichever element to be measured, and which responds in a known and predictable manner to fluctuations in the measured variable. Generally speaking, a sensor can be divided into two parts:
1) Probe
2) Power and/or signal conversion-adaptation electronics.
The probe, which is the element placed in the vicinity of the area to be measured, can in turn be divided into:                The sensor element, which generates a change in a property which can be measured as a function of the variation of the measurement variable.        Support element(s), whose function is to mechanically secure the sensor element to the area where the measurement is performed; in addition, in certain cases it may serve as an thermal or electronic insulator to ensure suitable working conditions for the probe.        
The probe also requires complementary electronics which can translate the signal arriving from said probe into a comprehensible figure expressed in appropriate units.
In order to determine O2 and CO2 levels, or those of any other components of combustion, probes are known which are placed in the exhaust ducts of vehicles powered by combustion engines.
In this sense can be cited European Patent EP-0448817, particularly conceived for application for exhaust pipe gases, where existent pressures are low, so that the mechanical design of the probe object of this European Patent does not allow its use in ducts which simultaneously have high temperatures and high pressures.
An oxygen sensing unit is described in document U.S. Pat. No. 5,049,255, which comprises a first and a second metallic bodies axially screwed onto each other. This sensing unit is specially designed for sensing oxygen in automotive exhausts, but would not perform efficiently in ducts under conditions of extreme temperature and pressure.